As conventional A/D converters, time A/D converters, referred to as TAD converters, have been developed. Examples of such TAD converters are disclosed in U.S. Pat. No. 5,396,247 corresponding to Japanese Patent Application Publication No. H05-259907.
TAD converters are each provided with a ring oscillator operating on an input analog voltage signal, and achieve high-resolution with a simple circuit structure.
TAD converters have normally nonlinear output characteristics that vary with variation in temperature. For this reason, in order to grasp a value of an input analog voltage signal based on the output of a TAD converter, it is desired to associate a variable of the TAD-converter output with that of an input analog voltage signal in accordance with its nonlinear input-output characteristic while the temperature dependence of the TAD-converter output is considered.
U.S. Pat. No. 6,891,491 corresponding to Japanese Patent Application Publication No. 2004-274157 discloses a method for approximating the input-output characteristic of a TAD converter. The method approximates the input-output characteristic of a TAD converter using polygonal line based on output values of the TAD converter corresponding to reference voltages in the input analog voltage signal within a preset input range, such as the upper limit, lower limit, and median value therein.